The present invention relates to tow trucks, for towing a disabled automobile, and more particularly to tow trucks which engage and lift the two front wheels or the two rear wheels of an automobile.
Tow trucks, for towing disabled automobiles by lifting either the front or rear wheels off the ground, and towing the partially-lifted automobile on its remaining two wheels, have long been used. Early versions of the tow truck provided a crane on the rear of the tow truck with a chain or cable having one or more hooks at their ends, which hooks engaged a part of the towed automobile adjacent the front or rear wheels, and lifted that part. Frequently, the hooks were attached to the chassis or, if the bumper was sufficiently securely attached, the hook may have been attached to the bumper. An example is Gaumont, U.S. Pat. No. 3,285,443. These hook-type wreckers or tow trucks were found with a change in automobile construction to be undesirable because there was danger of damaging the towed automobile, and as a result the so-called "sling" tow trucks were developed. In the sling-type tow truck, hooks were provided which engaged a part of the automobile, such as the axle, and these were connected through belts of flexible material, such as a rubber material, which could engage a part of the automobile, without damage: these belts were attached to a crane which could be raised and lowered, to thereby raise the front or rear end of the automobile. An example of a sling-type tow truck is Bubik, U.S. Pat. No. 3,620,393.
Another, and increasingly popular type of tow truck is that which is known as a "wheel-lift;" in this type of tow truck, the lifting of one end of the automobile is accomplished by engaging the wheels (tires) of the automobile at either the front or rear end of the automobile, and by raising the wheels raise that end of the automobile.
In a first type of wheel-lift tow truck, the tow truck has a crane which is secured to the tow truck behind the cab, and extends upwardly and rearwardly, a cable being connected to a winch and extending over a pulley or sheave on the end of the crane arm. A structure at the rear of the tow truck provides wheel-engaging elements, sometimes collectively referred to as the "wheel grid" and a connecting structure is provided between the wheel grid and the rear of the tow truck, the cable being attached to the connecting structure and effecting raising of the wheel grid and the automobile wheels upon actuation of the winch. Examples of this type of wheel-lift tow truck include Nelson, U.S. Pat. Nos. 3,051,337; Wagner, 3,182,829; Nelson, 3,434,607; Gaumont, 3,690,482; Pigeon, 3,924,763; Haring, 4,034,873; Peterson, 4,384,817; and Cannon, Jr. et al., 4,451,193.
The foregoing type of wheel-lift tow truck requires a crane mounted on and extending over the rear deck of the tow truck, thereby preventing the utilization of the rear deck of the tow truck, that is, the space between the rear of the cab and the rear bumper, from supporting large objects which it may be desirable to carry on the tow truck. In addition, the appearance of such tow trucks is cluttered and unattractive, and such tow trucks prevent unobstructed viewing of the towed vehicle by the driver while in the driver's seat of the truck.
A second type of wheel-lift tow truck has a crane including an arm or boom which is pivotable about a horizontal axis, and is actuated by a motor, typically a linear hydraulic motor. Examples of such constructions include Bubik, U.S. Pat. No. 3,897,879; and Place, U.S. Pat. No. 3,908,842. A related construction is disclosed in Bubik, U.S. Pat. No. 3,620,393, a sling-type tow truck. In general, these tow trucks have the same deficiencies as noted in connection with the tow trucks of the first type.
A third type of wheel-lift tow truck has a wheel engaging grid, and a boom which extends forwardly from the grid to a pivotal connection about a horizontal axis located substantially above the chassis and just rearwardly of the cab, and extending rearwardly beyond the rear of the tow truck. This construction is exemplified by LoCodo, U.S. Pat. No. 4,264,262; and Brown, U.S. Pat No. 4,473,334.
A fourth type of wheel-lift tow truck is known, in which the boom and related lifting mechanism are located beneath the deck of the tow truck; see Lind, U.S. Pat. No. 4,473,237. In this construction, a boom has its inner end pivotally mounted adjacent the rear axle, has a wheel grid at its outer end, and is rotated upwardly about its inner end by a pair of linear hydraulic motors which extend substantially vertically downwardly, although at a slight inclination to the vertical, from a pivotal attachment somewhat rearwardly of the rear wheels of the tow truck. This construction has a number of deficiencies, as explained below.
The basic truck structure which is used for constructing tow trucks is known as a one-ton truck, and is also known as a 10,000 pound gross vehicle rating truck. These have a uniform distance between the rear axle and the rear bumper. The towing apparatus should be constructed so as to lift the automobile without damage to it. Since, as shown by the above-discusses prior art, a boom or equivalent is used to connect the wheel grid to the tow truck, there is the possibility of damaging the towed vehicle by engagement of a part of it by the wheel grid and/or by the boom. In some types of vehicle, such as vans and Jeeps, the danger is minimal since there is a very short distance between the bumper and the adjacent wheel axle, and therefore there is no part of the van or Jeep which is in danger of being engaged by the wheel grid or boom. At the other extreme are the vehicle constructions which offer the greatest likelihood of damage to the vehicle by the lifting apparatus; these constructions are those in which there is a very substantial overhang of the body or chassis, beyond the rear and/or front axle. A Chevrolet Corvette is an example of a currently manufactured automobile which has a very long overhang at both the front and rear. This latter type of automobile requires that there be a very substantial reach of the boom, measured from the center of the tow truck rear axle to the mid-point of the wheel grid: typically, a wheel grid has a first part which engages the lower forward part of a tire, and a second part which engages the lower rearward part of the tire. The reference point of the wheel grid, therefore, will be between these two parts, and is directly below the axle of the lifted wheels. To provide the required maximum reach, a boom is utilized. Maximum reach is therefore one significant factor in the construction of a tow truck.
Another factor which must be taken into consideration is the possibility that the part of the weight of the towed automobile which is borne by the tow truck may tend to tip the front of the tow truck upwardly about the rear wheels. The principle elements influencing such tipping are the amount of weight borne by the tow truck, and the location of that weight relative to the rear axle. The greater the weight acting upon the tow truck and the further it is from the rear axle, the greater willl be the tipping moment generated by the towed automobile. The tipping moment may be even greater while towing, when dynamic forces generated by road irregularities increase the tipping forces. Consequently, to prevent the tow truck from tipping about its rear wheels--particularly while towing--the wheel grid should be as close as possible to the rear bumper of the tow truck, without risking damage to the automobile by engagement with the tow truck during turning, in which position the towed automobile is at an angle to the tow truck, and one corner or the other of the towed automobile is in danger of engaging an adjacent corner of the tow truck. To achieve minimum reach, the boom is made telescopic.
With a two-section boom as heretofore used, once the length is determined for maximum reach, the close-in or minimum reach is greater than desirable, considering the tow truck tipping moment. Thus, the construction disclosed in Lind, U.S. Pat. No. 4,473,237, is able to achieve either suitable maximum reach, or suitable close-in reach, but not both, and so has a limited range of the reach thereof. Parenthetically, in Brown, U.S. Pat. No. 4,473,334, a counterweight tank is provided at the front bumper of the tow truck to counteract the tipping moment, this tow truck having a two-section boom.
Although the range of movement of a retractable boom can be increased by utilizing a three-section boom, this solution is undesirable for the reason that the three sections tend to have "play" in them after a period of use. Consequently, when a bump or other road irregularity is encountered, unwanted raising and lowering movement on the wheel grid results in an action which is somewhat enhanced due to the fact that in the wheel-lift type tow trucks the sprung suspension system of the towed automobile is operative. As a result of either or both of these factors, up and down loads are generated which are undesirable. In addition, a three-section boom is more expensive than a two-section boom. To construct a three-section boom which will not have "play" requires additional strength, and a construction that is heavier and more expensive.
A further deficiency of Lind, U.S. Pat. No. 4,473,237, is that when the boom is in the towing position, it is inclined, providing some inclination for the wheels to roll out of the wheel grid.
The wheel-lift tow trucks of the prior art have required maneuvering of the entire tow truck in order to position the wheel grid properly in relation to the wheels of the automobile to be towed. This is because the boom has most frequently extended rearwardly from the tow truck and has been moveable only about a horizontal axis, as in Peterson, U.S. Pat. No. 4,384,817, and Cannon, Jr. et al., U.S. Pat. No. 4,451,193. However, some constructions of the first type above mentioned have provided not only vertical swinging movement of the boom about a horizontal axis, but in addition have provided horizontal swinging movement about a vertical axis, examples of such construction being Gaumont, U.S. Pat. No. 3,285,443 (a hook-lift tow truck); Gaumont, U.S. Pat. No. 3,690,482; and Pigeon, U.S. Pat. No. 3,924,763. In these constructions, however, the weight of the boom and grid are supported by a depending chain or cable which limits the ability of the boom to swing horizontally; i.e., the boom must rise as it is being swung on the vertical axis, unless the operator is able to pay out the chain or cable the amount necessary to keep the grid at the selected elevation while it is being swung horizontally. Such constructions in practice have not permitted the ready positioning of the wheel-grid, without maneuvering the entire tow truck to position the wheel grid; the truck is required to be placed in advance of the vehicle to be towed, a positioning that is often not possible in practice.
The wheel grid constructions heretofore used have provided a pocket for receiving a wheel (tire) of the towed vehicle. Examples of such construction include Nelson, U.S. Pat. No. 3,434,607, in which inclined plates engage the front and rear lower portions of the wheel; Gaumont, U.S. Pat. No. 3,690,482, which provides a pan with a forward portion engaging the lower forward portion of a tire, together with a hook for the axle; Pigeon, U.S. Pat. No. 3,924,763, which provides a rearward surface tire engaging element which is moveable when the wheel grid is raised or lowered; and Haring, U.S. Pat. No. 4,034,873, which provides front and rear inclined surfaces, etc.
LoCodo, U.S. Pat. No. 4,264,262, provides a wheel grid in the form of a wedge, which is caused by extension of the boom to move under and lift two automobile wheels, by wedging action; a suitable block is then placed on the wedge at the rear of each of the wheels, the wedge having holes for receiving pins extending from each block. In another construction, Brown, U.S. Pat. No. 4,473,334, the wheel grid includes U-shaped elements pivoted to a crossbar on a vertical pivot, swinging into position when one portion of the U-shaped element engages the front of a tire, upon extension of the boom. The constructions of the prior art do not provide for movement of the wheel of the towed vehicle in an upward direction without movement of the boom, thereby risking engagement of a part of the automobile with the wheel grid or boom when the boom is raised, and the wheel grid moves upwardly relative to the chassis of the automobile, due to such factors as deflection of the automobile sprung suspension system and changing of the shape of the tires due to the different load points caused by the wheel grids engaging the tires at positions upwardly from the part of the tire which engages the ground.